Hepatitis C virus (HCV) infection is a global health problem with 130-170 million individuals infected worldwide and 3.2 million people infected in the USA. The majority of those infected develop chronic HCV infections, which are a leading cause of liver failure and hepatocellular carcinoma in the US. IFN--based treatment is currently the most effective therapy for HCV infection. Unfortunally, ~50% of HCV patients do not achieve sustained virological response (SVR; ie., viral clearance) during treatment. So, understanding the mechanisms by which IL28B genetic variation modulates clearance of HCV will help us better understand how the IFN lambda system controls viral infection. IL28B (IFN-lambda3) is a member of the recently discovered type III interferon (IFN) family and single nucleotide polymorphisms (SNPs) in the IL28B promoter have been recently shown to strongly associate with spontaneous and treatment-induced clearance of HCV. Among those SNPs, polymorphism rs12979860 C/T is the strongest predictor of SVR to IFN--based treatment in patients with HCV infection. During HCV infection, patients with SNP-C produce more IL28B and have much higher rates of viral clearance compared with patients harboring SNP-T. However, how genetic variations in IL28B gene contribute to different expression of IL28B and HCV clearance is unknown. We found that the expression of IL28B mRNA was significantly reduced in liver tissues of HCV patients and in hepatocytes harboring SNP-T compared with those carrying SNP-C. This differential expression of IL28B was also reflected at the promoter level. Most importantly, we found that a unique nuclear protein DNA binding complex strongly binds to SNP-T but not SNP-C in hepatocytes. Our preliminary data suggest that hepatocytes with SNP-T genotype fail to express high amounts of IL28B due to binding of an unknown transcription factor to IL28B promoter, contributing to the persistence of HCV infection. Since the underlying molecular mechanisms are largely unknown, we propose to determine the differential expression of IL28B in primary human hepatocytes and in liver tissues of HCV patients harboring different SNPs, and identify the unknown transcription factor responsible for the reduced expression of IL28B in hepatocytes carrying SNP-T. Our goals are to elucidate the molecular mechanisms by which IL28B is differentially regulated by genetic variation during HCV infection. This will help us understand how the IFN lambda system controls viral infections, which in turn may help improve antiviral therapies.